It is not that difficult to build it into your app, if you're already storing information about hosts, networking etc. All you're really doing is expanding the scope, netbox is a fine starting point if you're willing to start there and build your systems around it, but if you've already got a system (or you need to do anything that doesn't fit netbox logic) you're probably better off just extending it.

In this case railway will need to care about a lot of extra information beyond just racks, IP addresses and physical servers.

correct; I think the first version of our tool sprung up in the space of a couple of weekends. It wasn't planned, my colleague Pierre who wrote it just had a lot of fun building it.